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The Benefits and Drawbacks of Twin Turbos

October 31, 2012

In the mid to late nineties, twin turbos were the feature to have on many flagship performance vehicles from Japan. These vehicles boasted large amounts of power, superb acceleration, and extensive potential for tuning. Though, as time has passed, both the benefits and advantages of the twin turbo layout have become quite apparent.

Parallel Twin Turbochargers

Parallel mounted twin turbochargers are used on a variety of upscale performance and luxury cars alike. This design, most commonly found on V type engines, uses a turbo for each bank of cylinders, with both running full time. The purpose of this layout is to reduce the amount of piping required for the plumbing of the intake and exhaust systems, which can increase response and reduce turbo lag. To this end, parallel twin turbochargers also tend to be smaller than what you would typically find on a single turbo engine of similar displacement. Because there is less exhaust gas available to each turbo, a smaller turbo must be used to maintain quick response, and the cost for this response, is limited high end power delivery and boost potential. For those seeking power, this can be the parallel twin turbocharger's greatest weakness next to cost.

Twin Sequential Turbocharger

A sequential twin turbocharged layout generally uses a paired combination of a small turbocharger for low end response, and a larger turbocharger for high rpm power. To avoid lag problems, the smaller turbocharger maintains quick response at lower engine speeds and also helps to spool the larger turbocharger via the increased exhaust flow. At higher rpm or at a factory determined switch over point, the workload is shifted to the larger, secondary turbo, allowing for high boost without the presence of the lag that would be apparent with a single large turbocharger. While sequential turbochargers help bridge the gap between top end power and low end agility, they tend to be overly complex, and in the case of the Mazda RX-7 Twin turbo, can even contribute to reliability concerns. In addition, for the custom tuner, the profiles of the smaller and larger turbocharger can be quite difficult and expensive to properly match. Because of this, many enthusiasts will simply upgrade to the simplicity of a larger, single turbo.

Twin turbo charging should not be confused with twin charging, which consists of the use of both a supercharger and turbocharger plumbed inline of one another. Twin turbo charging deals only with the use of two turbochargers, either in a sequential or parallel layout. Both of these layouts tend to suffer from large maintenance and modification costs, but besides this point, each has its own unique strengths and weaknesses. With parallel twin turbochargers, the biggest downside is likely the limited high rpm power, while sequential twin turbo setups suffer mainly from the complexity inherent of their designs. For both of these designs, single turbocharger kits are generally sold as upgrades due to their reduced complexity, higher potential power and improved reliability. For those who are willing to deal with the trouble and expenses however, a properly matched sequential turbocharger may be hard to beat.

Related Questions and Answers

What Is a Water Cooled Intercooler?

A water-cooled intercooler is used in a turbocharging system to help keep the fuel-air charge as dense as possible. Often exposed to long runs of heat generated by the engine, the normal fuel-air charge doesn't remain dense for long and this impacts turbocharger performance. A water-cooled intercooler sits on the "cold" water side of the radiator system, often having its own radiator fins, and this system helps to ensure that the fuel-air charge remains cool and dense for maximum power output.

How Does an Oil Cooler System Help a Car Intercooler?

An oil cooler system, similar in concept to a car's engine radiator system, helps an intercooler by taking the heat away from the engine. As much as the engine's oiling system works as a lubricating system - always a key component of any engine system - the oiling system also works as a cooling system. It moves much of the heat built up by the oil to the water jacket, and then out into the car's normal cooling system. An oil cooler, (by placing cooling fins or added cooling surface at or near the intercooler,) helps to keep the air-fuel charge dense by using the oiling system to remove excess heat.

What is the Ideal Temperature for Car Air Intakes?

Car air intakes work best when the fuel-air charge delivered to the fuel injector body is as cool and dense as possible. The best range of temperature for air-fuel charge should be in the range of 50-90 degrees Fahrenheit, as this is an almost perfect temperature to assure the best performance possible. Unfortunately, although this may be the temperature at the air intake, by the time the air-fuel charge works its way into the cylinders, it about doubles. This means that it is not dense, which is the key to engine performance.

What's the Difference Between a WasteGate and a Blow Off Valve in a Turbocharger?

A blow off valve on a turbocharger is designed to blow off or release the pressure that has built up in the intake manifold as you let off the throttle. Without a blow off valve, this pressure would end up back feeding to the turbo casing. This would cause the impeller to slow down, making getting back up to full power when you hit the throttle again slower. This is called turbo lag. The waste gate regulates the amount of boost the turbo puts into the motor. A turbo builds boost via a spinning impeller. When the PSI reaches a certain level, the waste gate opens, releasing the exhaust. When it falls back to a pre-determined level, the waste gate closes again. A simple way to remember is that a blow off valve regulates the pressure on the intake side of things, and the waste gate regulates it on the exhaust side.

Is a Supercharger Blow Off Valve the Same as a Turbocharger BOV?

Yes, a supercharger blow off valve is the same as a blow off valve on a turbocharger. The blow off valve vents the boost pressure between the turbo or supercharger and the intake manifold. This helps to alleviate compressor surge. This is the build up of pressure when you lift off the throttle under boost. The pressure will back up into the turbo or supercharger, which will slow the impeller, causing turbo lag. The blow off valve releases this extra pressure into the air or back into the compressor inlet depending on how it is set up. As it purges, it will make a 'pssst' noise. The blow off valve performs the same function on a supercharger and a turbo charger.

What are the Most Common Symptoms of Turbo Lag?

There is really only one main symptom of turbo lag, and that is the hesitation felt between the time you step on the accelerator, and when the turbocharger kicks in to boost. Turbo lag is inherently built into the system, as they are designed to operate at very specific RPM ranges. Air flow is very important to a turbo charger and how fast exhaust manifold pressure builds affect the turbo. Turbo lag will occur at different RPM levels in different cars, so no two cars will experience the same type of lag. The same can be said for correcting or minimizing lag. Each car will require a specific solution. Turbo lag can never be completely removed, but it can be reduced.